This invention is related to stressed-skin inflatable support systems which are exceptionally stable under axial and transverse loading conditions. The inflatable support systems of the invention are particularly useful as target supports for radar cross section (RCS) measurements.
Although various papers have appeared which are concerned with the stability of thin-skin shells, none are considered very well suited for supporting heavy loads or as radar target supports providing a low radar cross section suitable for radar scattering measurements.
For example: Weingarten, V. I., in his paper "Stability of Internally Pressurized Conical Shells under Torsion", AIAA Journal, Vol. 2, No. 10, pp. 1782-1788, October 1964, describes experiments with pressurized Mylar.RTM. conical shells to determine the effect of internal pressure on the buckling stress of such shells under torsion. Weingarten found: "It is evident that there is a large scatter band for the cone data, the average being about 88% of the theoretical value with the extremes ranging from 67 to 122% of the theoretical value." His experiments showed: ". . . yielding of the cone material near the small end as the pressure was increased." and went on to further state: "The quantitative agreement between Eq. (3) and the experimental results is poor, however, for conical shells . . . ".
In a later paper by Weingarten, V. I., et al, titled: "Elastic Stability of Thin-Walled Cylindrical and Conical Shells under Combined Internal Pressure and Axial Compression", AIAA Journal, Vol. 3, No. 6, AIAA Journal, pp. 118-1125, June 1965, the authors describe tests on pressurized cylinders and cones constructed of Mylar.RTM. under internal pressure and axial compression. The results indicated that the end-support and sealing methods were the main causes of failure (i.e., deformations appeared, buckles developed, and the onset of plasticity) which develop at or near the ends. As stated in their paper: "The scatter appeared to be dependent upon the end conditions, among other factors, since the two casting materials used, Cerrobend.RTM. and Cerrolow.RTM., gave consistently different results."
The earliest known paper on inflatables as target support for radar cross section (RCS) measurements is a report by Senior, T. B. A., et al, entitled: "Radar cross section target supports-Plastic materials", Rome Air Development Center, Griffiss Air Force Base Technical Documentary Report No. RADC-TDR-64-381 (Rome Report), June 1964. The report describes structural analysis and technical considerations of air bag target supports of various shapes, such as a simple truncated cone, a double truncated cone, and a cone cylinder combination. The simple conical shape was considered to be the most practical. "It was also recognized that the top of such a support will tend to balloon out into a hemispherical shape, which may pose mounting problems for certain types of targets. The ballooning can be overcome by properly designed Styrofoam.RTM. saddles, which will provide the necessary stability and attitude control."
A truncated cone, ". . . 16 feet in diameter and 30 feet high, fabricated from neoprene coated nylon with sewn seams was tested. It proved to be very stable, moved less than six inches in a forty knot wind. The support was inflated to a pressure of 0.25 psi. It was used to elevate a 150 pound target. Its theoretical capacity at the inflated pressure was estimated to be 250 pounds."
As stated in the Rome Report, ". . . the investigation of (1) Styrofoam structural properties, (2) low cross section structural bonds, and (3) the feasibility of air-inflated target supports. These investigations were not completed due to diversion of contract funds to more promising R & D areas."
The following year, Freeny, C. C., in his paper "Target Support Parameters Associated with Radar Reflectivity Measurements", Proceedings of the IEEE, Vol. 53, pp. 929-936, 1965 mentions the Rome Report. Sixteen years later, the only structures mentioned as useful to support targets for radar measurements (mentioned in "Radar Cross Section Handbook", by Ruck, George T; et al, Plenum Press, New York, pp. 915-923, 1970) were cellular plastic columns or dielectric suspension lines. Eighteen years later, Bachman, C. G., in his book titled "Radar targets", Lexington Books, Lexington, Mass.: D. C. Heath and Company, page 123, 1982 describes conventional methods of supporting targets such as polyfoam, steel column, and rope or string and inflatables as exotic and useful for supporting small targets. Twenty five years later, Knott, E. F., (in Chapter 9 on Far Field RCS Test Ranges of Nicholas Currie's book titled "Radar Reflectivity Measurement: Techniques & Applications", Artech House, Inc., Norwood, Mass., pp. 307-367, 1989) mentions three standard methods of supporting targets exposed to instrumentation radars for RCS measurements: plastic foam columns, strings, and absorber-coated metal pylons.